It is well known that most plastic injection molding machines have a heavy frame supporting a plurality (usually four) of horizontal tie bars which accommodate relative movement between die plates or platens respectively supporting the two half elements of a mold. A first such platen is supported in a fixed location on the tie bars and is referred to as the fixed platen, while the other platen is movable toward and away from the fixed platen along the tie bars. The platens support the mold halves so that they will become engaged and define the mold cavity when the movable platen is driven toward the fixed platen to engage the complementary mold halves. The mold halves are engaged under a heavy force load to prevent separation of the mold halves when plastic material is injected in the cavity, and the machines are often rated in tons, this giving an indication of the closing force load within the capacity of one machine as compared to that of another machine.
The lockup or mold engaging force can be read directly if the movable die platen is hydraulically driven, i.e., by a reading of the hydraulic force employed. However, in those injection molding machines wherein the movable die platen is reciprocated on the tie bars by mechanical linkages, there is no means of obtaining a direct reading of the tonnage or load applied when the mold halves are engaged.
In the past, a tonnage or force reading on the mechanical linkage machines was obtained by drilling a deep hole along the axis of a tie bar and using an indicator coupled to a long sensing rod thrust into the hole to indicate the stretch of a tie bar when load tonnage is applied. A deep hole in the tie bar being used is necessary in order to provide enough total stretch to give a usable displacement for the indicator reading. In such installation, the readout is tied to a specific length of bar being measured, that is the length or depth of the hole, and the actual working length of the tie bar changes with molds of different dimension. As a result, inaccurate readings are provided for any but the mold for which the hole was initially made.